Complex, small, fir tree or bulbous root form geometry milling cutters are difficult to make, having a relatively small number of cutting edges—typically 3 to 5 cutting edges—and hence have correspondingly restricted cutting rates. A recent development of Applicants, is a milling cutter described in WO 2008/090301, but whilst providing substantial advantages in the cutting sphere for which it was designed, is impossible to make in complex profiles containing a mixture of concave and convex shapes on the periphery.
The result is that, in order to machine roots of desired profile for turbofan components, extensive broaching operations from roughing to finishing are unavoidable, involving all manufacturers, worldwide, in a substantial broaching machine investment programme, with capital costs typically £2-3 million per machine, resulting in substantial installation costs, maintenance costs, floor area requirements, machining time, and hence significant manufacturing costs.
Although it is self-evident that the rate of metal removal of course increases with the number of teeth of a rotary cutter, and a broaching tool has numerous cutting teeth for both roughing and finishing operations in a single tool, the performance of form milling cutters is, as indicated above, constrained by the number of teeth that can be manufactured within a given tool diameter.
Whilst the range of cutters of WO 2008/090301 starts to address this issue on parallel or ball nosed tools, the production of a range of form milling cutters of both dovetail and bulbous forms, with a substantially increased number of teeth/flutes has been impossible to date. This has resulted in current industry use, worldwide, of form milling cutters incorporating between 4 and 6 teeth of a given tool diameter of 25 mm as exemplified in FIGS. 1A to 1C in fir-tree form and in FIGS. 2A and 2B in bulbous form, with the current constraints limiting the number of teeth that can be produced in existing form milling cutters.
Because, in prior art proposals, clearance on each tooth is generated by the arc of the grinding wheel, the diameter of the grinding wheel is the limiting factor during manufacture of form milling cutters, which diameter defines the number of teeth that can be produced. This is due to the current manufacturing process, where, as the tool is rotated to produce the necessary clearance, the grinding wheel fouls the following tooth and thus damages the form. Reducing the diameter of the wheel therefore allows for more teeth to be produced, but is impractical beyond a certain diameter of grinding wheel. Therefore, restriction of the number of teeth in the cutter normally is unavoidable.
In US 2008/020600 (SASU) is described a ball nose cutter, of relatively simple geometry for the machining of integrally bladed rotors, rather than being a form milling cutter with which the invention is concerned, and is a cutter that scans across a shape and is tapered for ready re-grinding.
In WO 2009/070424 (TDY Industries) is described a landless i.e. sharp toothed, burr rather than a form milling cutter with which the invention is concerned, and produced by the method of U.S. Pat. No. 5,868,606 (Martin), the latter describing a ball end cutter, with grinding by a wheel having an axis of rotation different to the cutter axis of rotation.
In U.S. Pat. No. 6,164,877 (Kamata) is described a fir-tree, formed milling cutter with but three cutting edges, supporting the comments above on the inability of any manufacturer to produce more than 3, 4 or at most 6 cutting edges on a fir-tree etc root form milling cutter, such that all turbofan manufacturers have been obliged to include a broaching step.